The present invention relates to a transformer coupled gate drive circuit for power MOSFETs and, more particularly, to a gate drive circuit which sharpens the rise-time of the pulse output of a transformer.
Drive circuits for high power devices such as power MOSFETs and the like, which drive circuits are transformer coupled to user-generated control signals, offer many advantages. These include, for example, impedance matching, DC isolation, improved protection from high voltages, and either step-up or step-down capability. Transformer coupled drive circuits allow circuit designers to reference gate driving control signals to any desired potential instead of to the typically high potentials which are present at the driven power MOSFETs or in the gate drive circuits thereof. This facilitates interfacing to and driving of power MOSFET devices and further permits the use of easily generated signals such as those that might be obtained from an output port of a microprocessor and similar low voltage devices. Transformer coupling provides many other advantages, as is well recognized in the art.
Nevertheless, certain attributes of transformers interfere with the ability to fully capitalize on their advantages. For example, transformers can deliver only AC signals since the core flux must reset, i.e. reverse itself, each half cycle. This is referred to as the "constant volt seconds" requirement of transformers. In essence, according to this requirement, the product (absolute value) of the voltage multiplied by time, in each half cycle, must be equal. As the duty cycle of an input is varied, the constant volt seconds requirement results in large voltage swings on the transformer as the duration of reset pulses is adjusted to control the power MOSFET.
Also, the signal rise-time of transformer signals is comparatively slow, which can result in excessively delayed and power wasteful switching of power MOSFETs in certain applications. There are also limitations which arise from the special considerations which must be given to the core, insulation, etc. of transformers.
It is accordingly desirable to be able to provide a transformer coupled drive circuit for high power transistors, including power MOSFETs, which avoids the disadvantages of transformers, particularly the slow rise-time of transformer outputs.
The prior art is familiar with various driving circuits for high power switching devices, including several which provide transformer isolation. This prior art is exemplified by several United States patents and by an Application Note of the assignee of the present invention, the International Rectifier Corporation.
The Application Note (No. 950A, reproduced at pages i-75 to i-77 of the fourth edition of International Rectifier's HEXFET DESIGNER'S MANUAL, POWER MOSFET APPLICATIONS AND PRODUCT DATA) illustrates and describes a single gate driving transistor which is disposed between the secondary of a coupling transformer and the gate of a HEXFET.RTM. power switch. The circuit has a high duty cycle ratio of 1-99% and any desired voltage ratio. The coupling transformer provides, of course, electrical isolation and other benefits of transformer coupling.
U.S. Pat. No. 3,609,405 discloses a sharp rise-time and fall-time, high amplitude pulse generator. FIG. 1 of the patent illustrates a pair of series connected bipolar transistors 42 and 44 which operate in a push-pull fashion. A capacitor 60 charges and discharges and thereby drives the base of a transistor 62. Since the discharge path for capacitor 60 has a very low impedance, the base drive to transistor 62 is high causing it to turn on rapidly. This rapid turn-on of transistor 62 results in a step-output having a rise-time on the order of 10 nanoseconds.
U.S. Pat. No. 3,657,568 discloses a pulse shaping circuit using complementary MOS devices having their gates connected in common to a MOS transistor switch for switching power to a load coil. A drive circuit for the MOS devices comprises, as an input stage, a differentiator consisting of a simple R-C circuit for the differentiating input pulses derived from the frequency converter of a typical electronic-type watch. The differentiated input pulses are applied to the MOSFETs, which produce sharp, square pulses, which are in turn applied to a switching transistor, also, preferably, in the form of a MOSFET.
U.S. Pat. No. 4,002,927 discloses a pulse control circuit comprising a first FET, which enters a conductive state in response to an input pulse, a first time constant circuit charged with the first FET and thereby put in a conductive state, a second FET which is rendered conductive with a charging voltage of the first time constant circuit, and a second time constant circuit for discharging the charged electricity in the first time constant circuit when the second FET is put in the conductive state.
Additional representative art is described in U.S. Pat. Nos. 3,573,502 and 3,446,993.
However, the need remains for a more effective drive circuit which is particularly suited for converting slow rise time, transformer coupled gate drive signals into signals having very sharp rise-times for driving power MOSFETs.